The present invention relates to brakes used on, for example, commercial truck or trailer axles, and in particular to automatic slack adjusters which eliminate excess motion in a brake actuator mechanism used to apply the brake.
Over the life of the brake linings of a brake, such as a pneumatic drum brake used on commercial vehicle axles, as the brake's friction linings wear the clearance between the brake linings and their respective friction surfaces (for example, the inner surface of a brake drum) increases. This increasing clearance requires an ever-increasing range of motion from the brake actuator mechanism to move the brake linings from their rest position to the point at which the linings contact the friction surface.
It has become commonplace to include an automatic slack adjuster in the mechanical path between the brake actuator and the brake linings so as to eliminate excess lining travel slack as the brake linings wear. Such adjusters typically are: (i) located on a portion of a brake camshaft which is outside of the brake (typically splined to the camshaft); and (ii) coupled to a pushrod of a brake actuator such that when the brake actuator push rod is extended or retracted, the slack adjuster rotates about the longitudinal axis of the brake camshaft. An example of such a brake and slack adjuster arrangement is shown in FIG. 1 of U.S. Pat. No. 4,380,276. Thus, by extending or retracting the brake actuator pushrod, the slack adjuster causes the brake camshaft to rotate about its longitudinal axis, which in turn rotates a brake actuation cam affixed to the end of the brake camshaft located within the drum brake. The rotation of the cam either presses the brake linings into engagement with the brake drum inner friction surface or allows the brake linings to withdraw radially inward, away from the friction surface. Because the brake camshaft is used to rotate the cam which presses the brake linings radially outward, the brake camshaft is also known as the brake cam.
Automatic slack adjusters can be designed to transmit brake actuator force to the brake camshaft in the brake application direction with no relative motion between the adjuster and the brake camshaft. When the brake actuation force is withdrawn, if there is greater than desired distance between the brake linings and the brake drum friction surface, the slack adjuster is permitted to rotate relative to the brake camshaft an angular distance sufficient to remove some or all of this undesired slack, i.e., limiting the distance the brake linings withdraw from the brake drum friction surface so that the lining-drum clearance is maintained at a desired minimum.
Automatic slack adjusters as described above, where the slack adjuster rotates relative to the brake camshaft when the brake actuation force is withdrawn, are said to adjust on release. There is also the other category of automatic slack adjusters which rotate relative to the brake camshaft during the phase when the actuation force is applied, with no relative rotation when the actuation force is withdrawn, this category being said to Adjust on Apply.
In many automatic slack adjusters, a one-way clutch is used to accomplish the rotary adjusting movement, with a worm shaft located in the adjuster turning a worm gear (also known as a worm wheel) coupled to the brake camshaft. In one type of one-way clutch arrangement, the one-way clutch is coupled to the worm shaft through a toothed clutch or a friction clutch located coaxially with the worm shaft. A heavy coil spring or disc-spring pack biases the one-way clutch to keep it engaged so that a torque applied through the one-way clutch can turn the worm shaft. The worm shaft turns the worm wheel, which is coupled to brake camshaft, in order to decrease the brake lining clearance and thus compensate for lining wear. Examples of such arrangements are shown in prior art FIGS. 1-3, corresponding respectively to FIG. 4 of U.S. Pat. No. 4,380,276 (toothed clutch teeth 63), FIG. 3 of U.S. Pat. No. 5,327,999 (toothed clutch 8), and FIG. 1 of U.S. Pat. No. 5,664,647 (toothed clutch 14).
A further type of one-way clutch is a ratchet and pawl arrangement, in which a pawl has to be manually retracted to retract the brake linings. This design has the problem that if the operator does not remember to retract the pawl when manually servicing the brake, an attempt to retract the brake linings can result in damage to the one-way clutch.
Regardless of the type of automatic slack adjuster, typically an external extension of the worm shaft projects outside the automatic slack adjuster housing to permit manual brake lining clearance adjustment during the installation of the slack adjuster or of new brake linings (in FIG. 1, extension 57; in FIG. 2, extension 4′; in FIG. 3, extension 15). The extension usually is shaped as a square or hexagon to facilitate gripping and turning with a wrench or other tool. In order to advance the brake lining, the worm shaft must be rotated in a first direction (designated the clockwise direction for the purpose of this description). In order to retract the brake lining, the worm shaft must be rotated in the opposite, or counter-clockwise, direction.
When the external extension is rotated in the clockwise (advance) direction, the toothed clutch remains engaged, and the worm shaft rotates with little resistance from the one-way clutch permitting the worm shaft to rotate with little resistance. When the external extension is rotated in the counter-clockwise (retracting) direction, the one-way clutch is rotated in its “lock-up” direction, and therefore the toothed clutch coupling strongly resists rotation of the worm shaft. The strong resistance requires application of high torque loads to the external extension, up to the point at which the toothed clutch begins to slip, disconnecting the one-way clutch from the worm shaft.
The slipping of the toothed clutch in response to the application of a large torque to the external extension often results in damage to the one-way slack adjuster, for example, in the case of toothed adjusters mounted on the worm shaft, the undesired blunting of the teeth in the clutch. As these clutch teeth wear, the torque capacity of the automatic slack adjuster decreases, progressively reducing the useful service life of the automatic slack adjuster. Attempts have been made to reduce this undesired deterioration of the clutch teeth, for example, by altering the angle of the clutch teeth or rounding the tips of the teeth as shown in prior art FIGS. 4a-4b, corresponding to FIGS. 3-4 of U.S. Pat. No. 5,664,647. However, these slight teeth geometry changes have not been fully successful in addressing the wear concerns.
In view of the foregoing, it is an objective of the present invention to provide an improved automatic slack adjuster with superior manual adjustment provisions. In addressing these and other objectives, the present invention provides a solution to the problems of the prior art by providing for controlled disengagement of the one-way clutch teeth to permit withdrawal of brake shoes as an external adaptor part is manually operated without incurring damage to the one-way clutch components.
In one embodiment of the present invention, the one-way clutch includes a clutch wheel concentrically arranged about an end of the worm shaft, a hex wheel concentrically arranged on the worm shaft adjacent to the clutch wheel and engaging the worm shaft in a non-rotating manner, and a power spring arranged to press the clutch wheel into non-rotating contact with the hex wheel with sufficient force that during a brake application event, the clutch wheel and the hex wheel do not rotate relative to one another. The one-way clutch is also provided with a mechanism which facilitates retraction of the brake linings without damage to the one-way clutch during manual operation of the external extension. In this embodiment, the clutch wheel and the hex wheel are provided with a ball detent clutch arrangement between their respective contact faces, arranged such that when a predetermined torque applied to the external extension is exceeded, the ball detent clutch elements overcome the pressure applied by the power spring, pushing the clutch wheel and the hex wheel axially apart and permitting the hex wheel to rotate the brake lining-retraction direction on the balls of the ball detent clutch, independent from the non-rotating clutch wheel. Preferably, the predetermined torque which must be exceeded by the manual actuation of the external extension is a torque which is higher that that normally observed between the clutch wheel and the hex wheel during brake application operations. This ensures that the ball detent clutch does not permit the clutch wheel and the hex wheel to move relative to one another during normal brake application events.
A ball detent clutch includes a series of balls arrayed in a pattern such as a circle, located between two parallel members. When at rest, the balls reside in detents on at least one of the parallel members. A normal force (provided, for example, by a spring) is applied to bias the parallel members toward one another. If a torque is applied to one of the parallel members to rotate the members, the torque is transferred through the balls to the opposing the parallel member. The strength of the spring biasing the parallel surfaces toward one another governs how much torque may be applied before, at a predetermined torque, the tangential force on the balls cause the balls to push apart the parallel members and allow the parallel members to begin to rotate relative to one another. In one type of ball detent clutch, the balls reside in recesses or holes in one of the parallel members, with a portion of each ball extending above the surface of the face of the member so that they can engage corresponding detents in the opposite parallel member, and when the predetermined torque is exceeded, the balls rise out of the detents on the surface of the opposite member and permit the parallel members to move relative to one another while the balls rotate in place within their respective recesses or holes. Once the predetermined torque is exceeded, torque is no longer transferred from one parallel member to the opposing member, i.e., the clutch is released.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.